Low noise generation fan



Oct. 7,1969 L. GRAY 3,

ow norsm GENERATION m1 Filed June 13, 1968 /kVVf/Vfflk? 1 4666% 4 4/ JZM 5% H6 A/r United States Patent O US. Cl. 230120 6 Claims ABSTRACT OFTHE DISCLOSURE A configuration for attenuating fan noise created by flowdisturbances is provided by an inlet passage in the fan housing havingan area discontinuity therein.

BACKGROUND OF THE INVENTION Field of the invention This inventionrelates to the attenuation of noise and particularly to the attenuationof fan noise.

Description of the prior art The nature of axial flow compressor noiseis described in US. Patent No. 3,194,487, and in an article by S. M.Levin entitled Aircraft Noise--Can it be Cut? appearing in SpaceAeronautics Magazine, August 1966, pages 65 to 75. These reference-sdisclose that in an axial flow compressor, the greater the spacingbetween the inlet vanes and the rotor, and the greater the spacingbetween the rotor and the exit vanes, the greater the noise attenu-3,471,080 Patented Oct. 7, 1969 In accordance with the presentinvention, a fan which has greater noise attenuation than those known inthe prior art is provided by an inlet duct which has area discontinuitytherein, the area of said inlet duct either increasing or decreasing asthe flow proceeds downstream towards impingement on the fan blades.

The inlet discontinuity attenuates noise in accordance I with thefollowing equation:

ation; this is so because the increased distance between the vanes andthe rotating blades allows more dissipation of turbulent upstream wakesbefore impinging on pressure fields around the downstream blades andvanes. The above-mentioned references also disclose that fan noisegeneration is also a function of the relative number of In the prior fanart it had not been thought prudent to:

introduce area discontinuity into the inlet of a fan because of thesupposition that the pressure drop through the inlet would increase tosuch an extent that fan performance would fall below acceptable levels.Therefore, it was thought that the only manner in which to achieve noiseattenuation in a fan was to space the rotor a relatively great distancefrom the inlet guide vanes, to space the exit vanes a relatively greatdistance from the rotor, and to utilize a large number of vanes ascompared to the number of blades on the rotor. However, I have foundthat the principle of area discontinuity can be adapted to attenuate fannoise by introducing discontinuities into the inlet duct of a fan,thereby considerably reducing the noise generation of the fan, withoutsubstantially increasing the pressure drop across the inlet and,therefore, without adversely affecting fan performance.

SUMMARY OF THE INVENTION The object of the present invention is toprovide increased fan noise attenuation without substantially increasingthe pressure drop of the fan inlet or adversely atfecting fanperformance.

Noise transmission loss (attenuation):

l 2 10 log [1+ sln 10L] db.

where m=area change ratio -r=wave length of sound =distance from onearea change to the next Since the noise field in a fan environment israndom, sin kL=1. Therefore, for a fan the equation may be expressed asv 1010 [1+ (m db gm 4 m The area changes may be either gradual or stepchanges although it is believed that step changes may degrade fanperformance somewhat more than gradual changes. Ad' vantages will berealized using my invention with one area change in the fan inletalthough it may be desirable to utilize more than one area change(discontinuity) to achieve the desired amount of noise attenuation. Onthe other hand too many area discontinuities may degrade fanperformance. Therefore, in practice the optimum numberof'discontinuities will depend upon the particular fan, whether of thehigh efiiciency type, etc., and the. sizes of the areas and changesthereof will be determinedin a trade-off with the desired fanperformance.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of a preferred embodiment thereof as illustrated in theaccompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevation view of a fan.

FIG. 2 is a sectional view taken along line 2'2 of FIG. 1 schematicallyshowing a fan having an inlet with discontinuities therein in accordancewith the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 2, thereis shown a preferred embodiment of my invention. A fan housing 2.surrounds a fan rotor 4 which is disposed on a shaft 6 rotatably mountedin bearing 8. The shaft is driven by motor means (not shown). A radialinlet 10 is provided for bringing the gas into the fan. Gas enters theradial inlet 10 and then flows into an annular chamber 12 and thence tothe rotor 4 in a direction indicated by the arrows. Inlet guide vanes 14are disposed upstream of the fan rotor 4 to straighten the flow of gasprior to its impinging on the fan blades 16. The inlet guide vanes 14are preferably spaced from the rotor a distance S equal to or greaterthan the height of the rotor blades. Likewise, exit vanes 18 aredisposed downstream of the rotor 4 a distance of at least S therefrom.The above-described spacing allows the harmonies of blade passagefrequency to decay in the duct both upstream and downstream of the rotorblades. As mentioned previously, it is desirable to utilize a largenumber of rotor blades, and I have found that the optimum number ofvanes to blades is determined by the following equation:

V=2nB+1 where V=the number of inlet guide vanes and the number of exitvanes;

B=the number of rotor blades; and

n=harmonic order-=1, 2, 3 so.

It has been found that a configuration in accordance with the aboveequation gives the greatest decay of the harmonics of blade passagefrequency.

My invention comprises area discontinuity in the fan inlet incombination with the above-mentioned noise attenuation features. Forpurposes of this description of the best mode of carrying out myinvention two inlet area changes are shown. Or in other words, there arethree separate inlet regions in the fan inlet upstream of the rotor 4.At each area discontinuity (i.e., between regions I and II and betweenregions II and III) a backward travelling sound wave is generated. Thefractional portion of forward wave energy distributed to generate thebackward travelling wave results in an energy loss, and, therefore, adecibel reduction in the continuing forward wave.

While I have shown three distinct area regions, it will be obvious toone skilled in the art that one area discontinuity will achieve noiseattenuation. Furthermore, while the area change in the fan inlet ducthas been shown as decreasing from the inlet to the fan rotor, it isobvious that my invention will also work successfully when the areachange in the fan inlet duct increases from the inlet to the fan rotor.Therefore, it should be understood that various changes and omissions inthe form and detail of the preferred embodiment my be made thereinwithout departing from the spirit and scope of the invention which is tobe limited only as set forth in the following claims:

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

I 1. A configuration for attenuating fan noise created by flowdisturbances caused by fan vanes and fan rotor blades comprising a fanhousing having an inlet passage with inlet guide vanes disposed therein,a fan rotor disposed downstream of the inlet guide vanes and spacedtherefrom a distance of at least the height of the fan rotor blade, andexit vanes disposed downstream of the fan rotor and spaced therefrom adistance of at least the height of the fan rotor blade, and said inletpassage in the housing having an area discontinuity therein.

2. A configuration for attenuating fan noise as recited in claim 1,wherein said inlet passage area discontinuity is such that the area ofsaid inlet passage decreases from the inlet of the inlet passage towardsthe fan rotor.

3. A configuration for attenuating fan noise as recited in claim 1,wherein said inlet passage area discontinuity is such that the area ofsaid inlet passage increases from t e inlet of the inlet passage towardsthe fan rotor,

4. A configuration for attenuating fan noise as recited in claim 1,wherein said inlet passage area discontinuity is a step change in area.

5. A configuration for attenuating fan noise as recited in claim 1,wherein said inlet passage area discontinuity is a gradual change inarea.

6. A configuration for attenuating fan noise created by flowdisturbances caused by fan vanes and fan rotor blades comprising a fanhousing having an inlet passage with inlet guide vanes disposed therein,a fan rotor disposed downstream of inlet guide vanes and spacedtherefrom a distance of at least the height of the fan rotor blade, andexit vanes disposed downstream of the fan rotor and spaced therefrom adistance of at least the height of the fan rotor blade, and wherein therelationship of the number of inlet guide vanes and exit vanes to thenumber of fan rotor blades satisfies the equation V= 2nB+ 1 where V=thenumber of inlet guide vanes and exit vanes,

B=the number of fan rotor blades, and

n=harmonic order=1, 2, 3 co, and said inlet passage in the housinghaving an area discontinuity therein for the attenuation of fan noiseaccording to the equation.

1 1 2 10 log 1 where m=the area change ratio within said inlet passage.

References Cited UNITED STATES PATENTS 1,585,218 5/1926 Watanabe 253-391,693,352 11/ 1928 Schmidt 103-89 2,191,341 2/1940 Curley 253-392,339,602 1/ 1944 Hagen 253-39 2,735,612 2/1956 Hausmann 230-1322,918,254 12/ 1959 Hausmann 253-39 3,045,895 7/1962 Bowter et al.230-120 3,194,487 7/1965 Tyler et al. 230-122 FOREIGN PATENTS 52,778 7/1910 Switzerland.

226,203 7/1925 Great Britain.

368,160 3/ 1932' Great Britain.

545,474 Belgium.

OTHER REFERENCES Space and Aeronautics: Stuart, M. Levin, August 1966,pp. 75 incl.

HENRY F. RADUAZO, Primary Examiner US, Cl. X.R. 30*133

